DEFINITION. 



91 



of common terms used in a scientific 

 sense, are almost always of the kind 

 last spoken of : their main purpose is 

 to serve as the landmarks of scientific 

 classification. And since the classi- 

 fications in any science are continually 

 modified as scientific knowledge ad- 

 vances, the definitions in the sciences 

 are also constantly varying. A strik- 

 ing instance is afforded by the words 

 Acid and Alkali, especially the former. 

 As experimental discovery advanced, 

 the substances classed with acids have 

 been constantly multiplying, and by 

 a natural consequence the attributes 

 connoted by the word have receded 

 and become fewer. At first it con- 

 noted the attributes of combining 

 with an alkali to form a neutral sub- 

 stance (called a salt) ; being com- 

 pounded of a base and oxygen ; 

 causticity to the taste and touch ; 

 fluidity, &c. The true analysis of 

 muriatic acid into chlorine and 

 hydrogen caused the second property, 

 composition from a base and oxygen, 

 to be excluded from the connotation. 

 The same discovery fixed the atten- 

 tion of chemists upon hydrogen as 

 an important element in acids ; and 

 more recent discoveries having led to 

 the recognition of its presence in 

 sulphuric, nitric, and many other acids, 

 where its existence was not previously 

 suspected, there is now a tendency to 

 include the presence of this element 

 in the connotation of the word. But 

 carbonic acid, silica, sulphurous acid, 

 have no hydrogen in their composi- 

 tion ; that property cannot therefore 

 be connoted by the term, unless those 

 substances are no longer to be con- 

 sidered acids. Causticity and fluidity 

 have long since been excluded from 

 the characteristics of the class by 

 the inclusion of silica and many other 

 substances in it ; and the formation 

 of neutral bodies by combination with 

 alkalis, together with such electro- 

 chemical peculiarities as this is sup- 

 posed to imply, are now the only 

 differenticB which form the fixed con- 

 notation of the word Acid, as a term 

 of chemical science. 



What is true of the definition of 

 any term of science is of course true 

 of the definition of a science itself ; 

 and accordingly, (as observed in the 

 Introductory Chapter of this work,) 

 the definition of a science must neces- 

 sarily be progressive and provisional 

 Any extension of knowledge or altera- 

 tion in the current opinions respecting 

 the subject-matter may lead to a 

 change more or less extensive in the 

 particulars included in the science ; 

 and its composition being thus altered, 

 it may Easily happen that a different 

 set of characteristics will be found 

 better adapted as differentiae for de- 

 fining its name. 



In the same manner in which a 

 special or technical definition has for 

 its object to expound the artificial 

 classification out of which it grows ; 

 the Aristotelian logicians seem to 

 have imagined that it was also the 

 business of ordinary definition to ex- 

 pound the ordinary, and what they 

 deemed the natur^, classification of 

 things, namely, the division of them 

 into Kinds ; and to show the place 

 which each Kind occupies, as superior, 

 collateral, or subordinate, among other 

 Kinds. This notion would account 

 for the rule that all definition must 

 necessarily be per genus et differ en- 

 tiam, and would also explain why a 

 single differentia was deemed suflfi- 

 cient. But to expound, or express in 

 words, a distinction of Kind has 

 already been shewn to be an impossi- 

 bility : the very meaning of a Kind 

 is, that the properties which distin- 

 guish it do not grow out of one 

 another, and cannot therefore be set 

 forth in words, even by implication, 

 otherwise than by enumerating them 

 all : and all are not known, nor are 

 ever likely to be so. It is idle, there- 

 fore, to look to this as one of the 

 purposes of a definition : while, if it 

 be only required that the definition of 

 a Kind should indicate what kinds 

 include it or are included by it, any 

 definitions which expound the conno- 

 tation of the names will do this : for 

 the name of each class must neces- 



